156 MOLLUSKS. 



External Features. Study the external features of the 

 snail by careful inspection of live and active specimens 

 moving about in a glass vessel. Observe that the foot 

 is bilaterally symmetrical, that it tapers to an obtuse 

 point behind, that it is squarish or truncated in front, 

 and that its foremost part is marked off from the rest by 

 a transverse groove on the ventral side. Observe, on the 

 dorsal side of this part, a pair of long motile and retrac- 

 tile tentacles, 'and a pair of eyes appearing as dark spots at 

 their bases. On the ventral side of this part is the mouth, 

 easily seen in a specimen that is crawling on the side of 

 the glass vessel. 



Observe the long, flexible body, extending upward from 

 the foot into the spire. 



Observe that the mantle is thickened at its edge where 

 it lines the edge of the aperture. This part is the collar. 



Observe in snails, at the surface of the water, a conspicu- 

 ous orifice in the collar at the right side of the body. 

 This is the respiratory orifice. The mantle is pushed in 

 at this point, and distended within into a sort of bag- 

 like rudimentary lung, in the walls of which the minute 

 blood vessels are distributed. 



Habits. Study the actions of the snail. How is its 

 peculiar crawling, creeping, or gliding locomotion effected ? 

 Note that it moves with equal ease on the bottom, on the 

 sides, and in inverted position along the surface of the 

 water. It will be seen to come to the surface at more or 

 less regular intervals for a fresh supply of air, and may 

 sometimes be seen expelling a bubble of impure air before 

 taking in a fresh supply. It will be seen to close its respir- 

 atory orifice before going down again. Occasionally a 

 snail may be seen to let itself down from the surface 

 by a floating thread of mucus secreted by glands in the 

 foot. When a number of snails going in opposite direc- 



